An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a steering assist command value of an assist (steering assist) command based on a steering torque Th detected by the torque sensor 10 and a vehicle velocity Vel detected by a vehicle velocity sensor 12, and controls a current supplied to the motor 20 based on a current control value E obtained by performing compensation and so on with respect to the steering assist command value. Moreover, it is also possible to receive the vehicle velocity Vel from a CAN (Controller Area Network) or the like.
In such an electric power steering apparatus, conventionally, for example, as disclosed in Japanese Published Unexamined Patent Application No. H8-290778A (Patent Document 1), a system stability and sensitivity characteristics of road surface information and disturbance information are simultaneously designed by a robust stabilization compensating section within the control unit 100.
However, in such a conventional control device, since a reaction force in a case of steering performed in the vicinity of a steering neutral position is small, due to the influence of friction, it is difficult to accurately transmit the road surface information to a driver. Further, it is difficult for the conventional electric power steering apparatus to let a hysteresis characteristic between a steering angle and the steering force be a characteristic the same as that of a hydraulic power steering.
As an apparatus for solving such a problem, there is an apparatus disclosed in Japanese Published Unexamined Patent Application No. 2002-369565 A (Patent Document 2).
A gross outline of the apparatus disclosed in Patent Document 2 will be described with reference to FIG. 2 corresponding to FIG. 1. As shown in FIG. 2, the motor 20 for generating the steering assist torque of the steering apparatus is driven by a motor driving section 21, the motor driving section 21 is controlled by the control unit 100 indicated by a dashed-two dotted line, and the steering torque Th from the torque sensor 10 and the vehicle velocity Vel from a vehicle velocity detecting system are inputted into the control unit 100. In the motor 20, a motor inter-terminal voltage Vm and a motor current value i are measured and outputted.
The control unit 100 comprises a torque system control unit 110 indicated by a dashed line that performs a control by using the steering torque Th and a motor system control unit 120 indicated by a dashed-dotted line that performs a control relating to driving of the motor 20. The torque system control unit 110 comprises an assist amount calculating section 111, a differential control section 112, a yaw rate convergence control section 113, a robust stabilization compensating section 114 and an SAT (Self Aligning Torque) estimation feedback section 115, addition sections 116A and 116B, and a subtraction section 116C. Further, the motor system control unit 120 comprises a compensating section 121, a disturbance estimating section 122, a motor angular velocity calculating section 123, a motor angular acceleration calculating section 124, a motor characteristic compensating section 125, and addition sections 126A and 126B.
The steering torque Th is inputted into the assist amount calculating section 111, the differential control section 112, the yaw rate convergence control section 113 and the SAT estimation feedback section 115, and all of them input the vehicle velocity Vel as a parameter. The assist amount calculating section 111 calculates an assist torque amount based on the steering torque Th. The yaw rate convergence control section 113 inputs the steering torque Th and a motor angular velocity ω, and brakes a movement that the steering wheel whirls to improve the convergence of yaw of the vehicle. Further, the differential control section 112 enhances a control responsibility in the vicinity of the neutral position of the steering and realizes a smooth steering. Moreover, the SAT estimation feedback section 115 inputs the steering torque Th, a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, the motor angular velocity ω calculated by the motor angular velocity calculating section 123 and a motor angular acceleration α from the motor angular acceleration calculating section 124 to estimate an SAT, performs signal processing by using a feedback filter with respect to the estimated SAT, and provides the steering wheel with a suitable road information as a reaction force.
Further, a signal that is obtained in the addition section 116B by adding the output of the yaw rate convergence control section 113 to a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, is inputted into the robust stabilization compensating section 114 as an assist amount AQ. For example, the robust stabilization compensating section 114 is a compensating section disclosed in Japanese Published Unexamined Patent Application No. H8-290778 A, removes a peak value in a resonance frequency of a resonance system comprised of an inertia element and a spring element that are included in the detected torque, and compensates a phase shift of the resonance frequency that disturbs the responsibility and the stability of the control system. By subtracting the output of the SAT estimation feedback section 115 from the output of the robust stabilization compensating section 114 in the subtraction section 116C, an assist amount Ia capable of transmitting the road information to the steering wheel as the reaction force, is obtained.
Moreover, the motor angular velocity calculating section 123 calculates the motor angular velocity ω based on the motor inter-terminal voltage Vm and the motor current value i, and the motor angular velocity ω is inputted into the motor angular acceleration calculating section 124, the yaw rate convergence control section 113 and the SAT estimation feedback section 115. The motor angular acceleration calculating section 124 calculates the motor angular acceleration α based on the inputted motor angular velocity ω, and the calculated motor angular acceleration α is inputted into the motor characteristic compensating section 125. In the addition sections 126A, the assist amount Ia obtained by subtracting the output of the SAT estimation feedback section 115 from the output of the robust stabilization compensating section 114, is added to the output Ic of the motor characteristic compensating section 125, and then this added signal is inputted into the compensating section 121 comprised of a differential compensating section or the like as a current command value Ir. A signal that is obtained by adding the output of the disturbance estimating section 122 in the addition section 126B to a current command value Ira obtained by compensating the current command value Ir by means of the compensating section 121, is inputted into the motor driving section 21 and the disturbance estimating section 122. The disturbance estimating section 122 is an apparatus disclosed in Japanese Published Unexamined Patent Application No. H8-310417 A, is capable of maintaining a desired motor control characteristic in an output reference of the control system based on a signal obtained by adding the output of the disturbance estimating section 122 to the current command value Ira compensated by the compensating section 121 that is the control target of the motor output and the motor current value i, and does not lose the stability of the control system.
Here, the aspects of torques generated between a road surface and a steering will be described with reference to FIG. 3. When the driver steers the steering wheel 1, the steering torque Th is generated and then the motor 20 generates an assist torque Tm in accordance with the steering torque Th. As a result, wheels are steered and the SAT is generated as the reaction force. Further, in such case, due to an inertia J and a friction (a static friction) Fr of the motor 20, a torque becoming the resistance of steering the steering wheel, is generated. By considering a balance between these forces, and by setting a sign( ) as a sign function, a motion equation such as the following Expression 1 is obtained.J˜·+Fr˜sign(ω)+SAT=Tm+Th  [Expression 1]
Here, by setting initial values to zero, performing a Laplace transform for the above Expression 1 and then solving with respect to the SAT, the following Expression 2 is obtained.SAT(s)=Tm(s)+Th(s)−J−α(s)−Fr−sign(ω(s))  [Expression 2]
It is clear from the above Expression 2 that by preliminarily obtaining the inertia J and the static friction Fr of the motor 20 as constants, it is possible to estimate the SAT based on the motor angular velocity ω, the motor angular acceleration α, the steering assist torque Tm and the steering torque Th. For such a reason, the steering torque Th, the motor angular velocity ω, the motor angular acceleration α and the output of the assist amount calculating section 111 are inputted into the SAT estimation feedback section 115.
Further, in the case of directly feeding back an SAT estimation current value *SAT estimated by the SAT estimation feedback section 115 without any processing, since the steering becomes too heavy, it is impossible to improve the steering feeling. Therefore, as shown in FIG. 4, a signal processing is performed with respect to the SAT estimation current value *SAT by using a feedback filter 115A having a vehicle velocity sensitive gain and a frequency characteristic, and only necessary and sufficient information for improving the steering feeling is fed back. The feedback filter 115A used in here comprises a Q-filter (phase-lag) 115B having a gain as a static characteristic gain that reduces the amplitude of the estimated SAT to a necessary and sufficient value and a gain section 115C having a gain characteristic shown in FIG. 5 that is sensitive to the vehicle velocity Vel, and decreases the road surface information to feed back in the case that the importance of the road surface information such as a static steering or a low speed driving is relatively low.
Although the apparatus described in the above Patent Document 2 configures the feedback of SAT estimation so that a frequency band in which there are disturbances that need to be suppressed and a frequency band in which there are disturbances that need to be transmitted are compatible, there is no function that actively cancels out the disturbances that need to be suppressed.
On the other hand, in the vehicle, at ordinary braking and steady-state running, a brake judder and a shimmy that give annoyance to passengers occur. The brake judder is a floor and pedal vibration occurring at braking of the vehicle, and sometimes induces a vibration in steering rotation direction. A variation in braking torque caused by DTV (Disk Thickness Variation) of the brake disk is the excitation source of the brake judder and has the first-order and higher-order components of the wheel rotation. It is amplified by the fore-and-aft resonance of the suspension and transmitted through the vehicle body and the steering system, and ultimately becomes the floor and pedal vibration and the steering vibration. Further, the shimmy is a vibration that occurs in the steering rotation direction during running of the vehicle. An imbalance and non-uniformity of rotating parts such as tires and wheels become the excitation source of the shimmy. It is amplified by the suspension resonance and then becomes the vibration in steering rotation direction through the steering system.
The apparatus of Patent Document 2 does not consider the brake judder and the shimmy described above at all. Further, in Japanese Published Unexamined Patent Application No. 2002-145075 A (Patent Document 3) and Japanese Published Unexamined Patent Application No. 2002-161969 A (Patent Document 4), although apparatuses that damp vibrations due to the brake judder and the shimmy are disclosed, both of which are mechanical handling, and there is a problem that the cost increases and simultaneously a finely-tuned suppression such as the vehicle velocity sensitive is impossible.
Furthermore, in the case that the inertia and the friction of the steering system are large, although the vibrations due to the brake judder do not spread to the steering wheel, for the sake of a good steering feeling and a vehicle stability, it is preferred that the inertia and the friction of the steering system are minimal.
In such an electric power steering apparatus, recently, vehicles equipped with a parking support function (parking assist) that switches an automatic steering mode and a manual steering mode appear. In a vehicle equipped with the parking support function, a target steering angle is set based on data from a camera (image), a distance sensor or the like, and an automatic control in accordance with the target steering angle, is performed.
In PCT Publication No. WO2008/146372 (Patent Document 5), the steered wheel is steered depending on driver's steering wheel operation by comprising a target driving amount calculating section that generates a target auxiliary steering angle or a target steered angle added by an auxiliary steering angle superposition mechanism based on a steering-wheel steering angle detection value from a steering wheel angle detecting section and a transmission characteristic, and calculates a target driving amount of a motor so as to make the target auxiliary steering angle and an auxiliary steering angle detection value from an auxiliary steering angle detecting section coincide with each other or so as to make the target steered angle and a steered angle detection value from a steered angle detecting section coincide with each other, and a motor driving section that drives the motor in accordance with the target driving amount.